275/50R20 vs 255/50R21
255/50R21 is a safe swap for 275/50R20 by the usual ±3% rule: the diameter differs by +0.69%.
Speedometer and odometer
With 255/50R21 on a car calibrated for 275/50R20, the speedometer reads 59.6 mph when you're really doing 60, and the odometer under-counts by 0.7%.
| Speedo shows | Actual speed |
|---|---|
| 30 mph | 30.2 mph |
| 45 mph | 45.3 mph |
| 60 mph | 60.4 mph |
| 70 mph | 70.5 mph |
| 80 mph | 80.6 mph |
Other sizes close to 275/50R20
- 275/50R20 vs 275/55R20+3.51%
- 275/50R20 vs 255/50R20-2.55%
- 275/50R20 vs 265/50R20-1.28%
- 275/50R20 vs 245/50R20-3.83%
- 275/50R20 vs 275/45R20-3.51%
- 275/50R20 vs 255/55R20+0.70%
- 275/50R20 vs 235/55R20-2.11%
- 275/50R20 vs 265/45R20-4.66%
- 275/50R20 vs 255/60R19+0.72%
- 275/50R20 vs 285/55R20+4.92%
- 275/50R20 vs 275/45R21-0.27%
- 275/50R20 vs 295/40R20-4.98%
Questions
Can I replace 275/50R20 with 255/50R21?
255/50R21 is a safe swap for 275/50R20 by the usual ±3% rule: the diameter differs by +0.69%.
What's the difference between 275/50R20 and 255/50R21?
255/50R21 is 0.21" taller and 20 mm narrower; the sidewall is -0.39". At an indicated 60 mph you'd actually be doing 60.4 mph.
Will 255/50R21 fit on 275/50R20 wheels?
No — 275/50R20 needs a 20" wheel and 255/50R21 a 21" wheel. This is a plus/minus-size swap that changes wheels too.